The present invention relates generally to shimming of magnets, and more particularly to passively shimming an open magnet.
Closed magnets have a single magnetic assembly with a bore in which is located the working magnetic field volume. Open magnets have two spaced-apart magnetic assemblies with generally coaxially aligned bores and a working magnetic field volume located in the open space between the magnetic assemblies. Open magnets have advantages in certain applications such as in MRI (magnetic resonance imaging) medical imaging where the open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. Real magnets have an inhomogeneity of the magnetic field in the working magnetic field volume due to manufacturing tolerances and site conditions. In many applications, the open or closed magnet must be shimmed to reduce the inhomogeneity of the magnetic field in the working magnetic field volume to within a predetermined specification. For example, an open MRI magnet whose magnetic assemblies are superconductive coil assemblies must be shimmed to reduce the inhomogeneity of the magnetic field in its working magnetic field volume, which is its imaging volume, to within a few parts per million for use in medical diagnosis.
Known methods for shimming closed superconductive MRI magnets include active shimming and passive shimming. Active shimming typically requires a complex arrangement of superconductive shimming coils. Passive shimming typically involves the placement of carbon steel shims of calculated thickness in the bore of the closed magnet at calculated locations on the inside diameter of the superconductive coil assembly. The thickness and location of the shims are determined through use of a computer shim code, as is known to those skilled in the art, which calculates adding shims to reduce the inhomogeneity of the mapped magnetic field in the imaging volume of the closed MRI magnet. The calculated shims are added to the magnet, the magnetic field of the magnet is again mapped, and the computer shim code is again run. This process is repeated until the inhomogeneity of the measured magnetic field in the imaging volume is reduced to within a predetermined specification. The repetitive nature of the shimming process is the result of the computer shim code being only an approximation of the real magnet.
Typically the shimming process starts with approximating the measured magnetic field in the imaging volume in terms of a Legendre polynomial expansion, as is known to those skilled in the art. For closed superconductive MRI magnets having magnetic field inhomogeneities, a typical Legendre polynomial approximation of the magnetic field within the working magnetic field volume would include Legendre polynomial terms (harmonics) up to order 8 and degree 8, including 2, 0 Legendre polynomial harmonics (i.e., the Legendre polynomial term of order 2 and degree 0). A typical computer shim code (as mentioned in the previous paragraph) defines a function related to the measured magnetic field inhomogeneity as affected by the addition of shims and then calculates, in an iterative manner, the thickness and location of shims to be added to the magnet which minimizes the defined function.
Applicant encountered problems in trying to passively shim open magnets. Shims could not be placed in the open space between the magnetic assemblies, and a typical open magnet could only be conventionally shimmed from a peak-to-peak inhomogeneity of about 1,000 ppm (parts-per-million) to about 400 ppm. Applicant discovered that adding shims only to the magnetic assemblies could not create positive 2, 0 Legendre polynomial harmonics which were needed to compensate for negative 2, 0 Legendre polynomial harmonics created when shims were added to reduce inhomogeneities of other harmonics. Open magnets were designed by others to have built-in positive 2, 0 Legendre polynomial harmonics such as by having a larger separation between the magnetic assemblies or by initially adding full shim trays and then removing some shims to create positive 2, 0 Legendre polynomial harmonics. However, when Applicant used a typical shimming computer code to add shims to reduce the inhomogeneity of the mapped magnetic field of such a positive-2, 0-biased open magnet, it was found that such open magnet could only be shimmed from a peak-to-peak inhomogeneity of about 1,000 ppm (parts-per-million) to about 50 ppm. What is needed is an improved method for passively shimming an open magnet to even lower levels of inhomogeneity.